French Patent Application FR 2 802 647, filed on 17 Dec. 1999 in the name of Thomson-CSF, discloses such a probe comprising a pitot tube for measuring the total pressure Pt of the airflow and means for measuring the total temperature TT in the form of a channel, the air entry orifice of which substantially faces the flow and of a chamber that includes a temperature sensor. The chamber takes off part of the air flowing in the channel.
The measurement of these two parameters—total temperature and total pressure—is used in determining the actual velocity of the aircraft. Advantageously, the multifunction probe furthermore includes means for measuring the static pressure Ps and the angle of incidence α of the airflow surrounding the aircraft. All the parameters needed to determine the modulus and the direction of the velocity vector of the aircraft are therefore available.
Measurement of the total temperature assumes theoretically that the streams of air for which the temperature is measured have a zero velocity. However, a certain amount of ventilation must be provided around the temperature sensor so as to prevent the latter from measuring the temperature of the probe structure. This ventilation also allows a sufficiently short response time to be achieved. The ventilation is generally provided by the speed of the aircraft, which drags air into the channel and into the chamber where the temperature measurement is carried out. On board an aircraft, the probe must operate within a wide velocity range, which results in substantial velocity variations in the chamber. Consequently, the measurement carried out by the temperature sensor is subject to an error, called the “recovery error”, which essentially depends on the velocity of the aircraft. At low velocity, the recovery error is low, but the response time is long. At high velocity, the temperature sensor is well ventilated and the response time is therefore shorter, but the recovery error is large.
Moreover, probes mounted on an aircraft are subjected to substantial temperature variations and, sometimes, to conditions under which ice may develop, especially inside the pitot tube or inside the channels for measuring the total temperature TT. The ice disturbs the measurements and, to prevent this, the probe includes means for heating it. These means generally comprise a wire that heats the probe by Joule heating. This wire is coiled in the walls of the probe. The power dissipated by the wire is of the order of several hundred watts. This power disturbs the measurement carried out by the temperature sensor, by heating the probe structure, and introduces an error called the “deicing error”, which is greater the higher the injected power.
Attempts have been made to correct this error by placing a heat shield around the temperature sensor and by optimizing the shapes of the entry channel and of the chamber so as to improve the airflow around the temperature sensor. For example, efforts have been made to remove from the probe, air forming part of a thermal boundary layer that develops along the wall of the channel and of the temperature measurement chamber. This boundary layer is particularly disturbed by the heating of the probe. Despite such precautions, it is barely possible to obtain a correct measurement of the total temperature for low aircraft velocities, especially for aircraft velocities below Mach 0.3. The thickness of this boundary layer is greater the lower the aircraft velocity. However, the measurement accuracy at such low velocities is of considerable importance for the safety of the flight as it makes it possible to determine the parameters of the airflow surrounding the aircraft, right from the taxing phase of the aircraft and before it takes off. To alleviate this problem, a forced circulation of air is produced in the measurement chamber by means of a venturi effect produced on the air stream escaping from the chamber. This solution is expensive as it requires considerable mechanical modifications. In addition, aircraft manufacturers insist on being able to carry out, especially on the new-generation large-capacity aircraft, total temperature measurements under increasingly severe icing conditions, thereby requiring greater deicing power levels and thereby further increasing the deicing error.